Fuse links for dropout expulsion fuses

ABSTRACT

A fuse link for dropout expulsion fuses (distribution cut out fuses) which comprises a vulcanized fiber tube having top and bottom arcing contacts secured in its ends and a fusible element extending between and held in electrical contact with said top and bottom arcing contacts. The fusible element comprises a plurality of thin silver wires which are coated with a mixture of a compound which burns and a compound which decomposes to produce oxygen at temperatures in the region of the fusing temperatures of silver. On fusion an arc is created and the coating explodes to ensure vaporization of the fusible element and also expulsion of the bottom arcing terminal.

United States Patent [191 Swarbrick et a1.

[54] FUSE LINKS FOR DROPOUT EXPULSION FUSES [75] Inventors: John Atkinson Swarbrick, Ringwood; Vaclav Puta Thornbury, Victoria, both of Australia [73] Assignee: Stanger & Co. Ltd., Victoria, Australia [22] Filed: Oct. 15, 1971 [211 App]. No.: 189,625

[52] U5. Cl. ..337/290, 337/231, 337/296 [51] Int. Cl. ..H0lh 85/04 [58] Field of Search ..337/159, 160, 231,

[56] References Cited UNITED STATES PATENTS 1,856,701 5/1932 Gerdien ..337/290 1,629,266 5/1927 Feldkamp 2,281,266 4/ 1942 Carlisle et al 2,504,901 4/1950 Storm et a1 1,867,924 7/1932 Roberts et al. ..337/29O X [451 May 22, 1973 FOREIGN PATENTS OR APPLICATIONS 10,130 9/1899 Sweden ..337/281 Primary Examiner-Bernard A. Gilheany Assistant Examiner-F. E. Bell Attorney-Cushman, Darby & Cushman [57] ABSTRACT A fuse link for dropout expulsion fuses (distribution cut out fuses) which comprises a vulcanized fiber tube having top and bottom arcing contacts secured in its ends and a fusible element extending between and held in electrical contact with said top and bottom arcing contacts. The fusible element comprises a plurality of thin silver wires which are coated with a mixture of a compound which burns and a compound which decomposes to produce oxygen at temperatures in the region of the fusing temperatures of silver. On fusion an arc is created and the coating explodes to ensure vaporization of the fusible element and also expulsion of the bottom arcing terminal.

10 Claims, 5 Drawing Figures P//// I u PATENTEDW 73 FUSE LINKS FOR DROPOUT EXPULSION FUSES FIELD OF THE INVENTION This invention relates to fuse links for expulsion fuses, and to fusible elements therefor, and especially, although not exclusively, dropout expulsion fuses (also called distribution cut out fuses).

BACKGROUND OF THE INVENTION The dropout expulsion fuse has been in use for many years and is the most inexpensive means for providing fusing on faults occuring at distribution voltages. There is a fire risk when expulsion fuses operate and many bush tires are known to have been caused by expulsion fuses.

Devices have been developed to reduce the risk of fires by catching the molten metal particles expelled from the fuse carrier. These devices are not 100 percent effective.

It is considered that the most effective way of preventing fires is to prevent the emission of molten metal particles and incadescent material from the fuse carrier. Accordingly it is the primary object of the present invention to provide a fusible element and fuse link which substantially reduce the emission of such particles and material and therefore reduces the fire risk and improves the performance and safety of expulsion fuses.

The conventional expulsion fuse link consists of a top button terminal, a short length of tinned copper braid, an upper arcing terminal, a strain wire element of high strength in parallel with a silver fuse element, the strain wire, fuse element and copper braid being crimped in the upper arcing terminal, and a lower arcing terminal crimped to the strain wire, fuse element and a lower flexible of tinned copper braid.

When this fuse element melts, particularly on low current faults about I MVA, molten particles of the silver and strain elements are expelled from the fuse carrier.

In an endeavour to find a solution to the problem of molten metal discharge, the inventors experimented with fuse links composed of fine silver fuse elements which tend to vapourize on reaching temperatures occuring at distribution voltage fault levels. It is known from U.S. Pat. No. 2,209,823, for example, to use vapourizable fine silver wires in fuses which are not required to cause a fuse carrier to dropout. However, such wires are usually immersed in a granular heat absorber and for this reason, and by virtue of the rigid construction of the fuse body, they are not suitable for expulsion fuses.

Accordingly, a more specific object of the invention is to provide a fuse link and fusible element therefor, which are suitable for expulsion fuses and which still achieve the primary object of the invention.

SUMMARY OF THE INVENTION The invention provides a fusible element for a fuse link adapted for use in an expulsion fuse comprising one or more silver elements of small effective crosssectional dimension which tend to vaporize, or form very small particles which cool rapidly, on fusion resulting from faults at current distribution voltages, the element or elements being coated or having arranged in close proximity thereto a compound or mixture which burns and which produces oxygen at temperatures approaching or at the fusion temperature or at temperatures which occur once an arc is produced by rupture of the element or elements to facilitate the vaporization of said element or elements.

The invention also provides a fuse link for a gas expulsion fuse comprising a top arcing terminal and a bottom arcing terminal, a fusible element electrically connecting said terminals, a tube or other member enclosing said fusible element and in the ends of which said terminals are received, means for securing said terminals in said tube or other member to allow the usual forces applied to said terminals to be supported by the tube rather than by the fusible element, said bottom terminal being expelled from said tube on occurrence of a fault, said fusible element being as defined in the above paragraph.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary cross-sectional elevation taken along a longitudinal center line of a fuse link embodying the invention;

FIG. 2 is a sectional end elevation of the bottom arcing terminal shown in FIG. 1, and

FIG. 3 is an elevation of a small section of the fusible element embodying the invention.

FIGS. 4 and 5 are sectional views of two further embodiments of a fusible element.

DESCRIPTION OF THE PREFERRED EMBODIMENT The fuse link described below is designed for dropout expulsion fuses of the type used in power distribution systems. Accordingly the terms top and bottom refer to the relative positions the parts of the fuse link assume when connected to a drop-out expulsion fuse in the position it assumes in use.

The fuse link shown in FIG. 1 comprises a top button head terminal 1 including a terminal 2 engaged by a dished button washer 3, a top arcing contact 4 having a narrow portion 5 received in a blind bore 6 in the terminal 2 and an enlarged portion 7 received in the top end of a fiber tube 8 of spirally wound vulcanized fiber material such as Leatheroid. The top arcing contact 4 has a central bore 9 through which a fusible element 10, to be described in detail below, passes to extend between the terminal 4 and the terminal 2. The contact 4 and the terminal 2 are rigidly connected, and the element 10 held in electrical contact therewith, by means of crimping the terminal 2 in a manner similar to the crimping shown in FIG. 2.

The arcing contact 4 is held in the tube 8 by a metal collet 11 which is a tight fit on the tube 8. The retention of the contact 4 is assisted by the provision of upwardly inclined peripheral ribs around the portion 7 which dig into the tube 8 when the collet 11 is crimped onto the tube 8.

The bottom end of the tube 8 holds a lower arcing contact 12 having an enlarged portion 13, also having peripheral ribs, and a narrow portion 14. Portion 13 has a countersunk bore 15 which opens into a bore 16 in portion 14 and which receives a tinned flexible copper braid tail l7 dip tinned at T to consolidate the end. The fusible element 10 passes through bores 15 and 16 and between the tail 17 and portion 14 and the contact 12, tail 17 and fusible element 10 are connected electrically and mechanically by crimping the portion 14 as shown in FIG. 2.

The contact 12 is retained in the tube 8 by an extruded plastic collet 18 which is a tight fit on the tube 8. Before the collet 18 is fitted, the tube 8 is double crimped about the portion 13 to ensure that the ribs on portion 13 dig into the tube 8. However, without the collet 18, the contact 12 is easily pulled from the tube 8. The collet 18 has some flexibility, and while the required tensile forces may be applied to the tail 18 in the process of fitting the link to an expulsion fuse and when actually fitted thereto without any risk of detachment of the contact 12, the flexibility of the collet 18 ensures expulsion of the contact 12 on the generation of gas under pressure on fusing of element 10 when a fault occurs. The top contact 4 is not normally blown from the tube 8 unless the fault is at a high level.

It will be appreciated that the tube and the top and bottom arcing contacts form an assembly which takes all of the mechanical forces applied to the fuse link when fitted to the fuse carrier of a dropout expulsion fuse and no unacceptable forces are applied to the fusible element 10 itself.

Referring now to FIG. 3, the fusible element 10 will be seen to comprise five strands of silver wire 20, each of which has a coating 21 over that portion of its length which is disposed in use in the tube 8. The free ends of the wires 20 are uncoated to facilitate good electrical contact with the arcing terminals 4 and 12. The coated portions of the wires 20 are adhered together before the coating dries to form the composite fusible element 10. The wires 20 are 0.003 inches in diameter.

In the present case, the coating comprises a mixture of four parts by weight of white BOSTIK (Registered Trade Mark) adhesive and five parts by weight of KC 10;, intimately mixed with a suitable solvent, such as lighter fuel, to form a slurry in which the KC10 is thoroughly dispersed in the adhesive. The BOSTIK adhesive is rubber based and burns readily at or near the fusing temperature of silver, and KCl is a compound which decomposes to produce oxygen at these temperatures. It is not known in which orderthese reactions occur since once the element fuses and an arc is produced, the temperature of the arc is in the region of 5,000C and the coating explodes to evaporate the element 10.

The production and rapid expansion of gases in the tube 8, produced by the decomposition and burning of the coating 21, by vapourization of the silver and by the consequential heating of the vulcanized fiber of the tube 8 which itself produces gases, causes rapid extinction of the arc and expulsion of the bottom arcing terminal 12. The expanding gases usually also rupture the tube 8 due to its spirally wound construction but not before sufficient pressure is built up to ensure rapid arc extinction. Fragments of the tube 8 are often expelled from the fuse carrier, but the arc duration is so small and the pressure of gases so high that these do not ignite.

It is essential that the bottom arcing terminal 12 be expelled as above since unless the fiber tube 8 breaks completely, and this is unusual other than at very high fault levels, the fuse carrier will not drop out. Thus, the design of the tube 8, the bottom arcing contact 12 and the ABS collet 18 is such as to achieve expulsion at the lowest fault level for which the fuse link is designed whilst allowing normal handling and the application of tensile forces encountered in assembly and use. In the present embodiment, the fuse link has been tested to a tensile force of about 50 lbs, the tensile forces encountered in use being of the order of 6 lbs.

The configuration of the lower arcing terminal 12 is such as to maintain the required strength whilst reducing its mass to a minimum. This, combined with the position of the terminal at the bottom of the tube and thus close to the bottom of the fuse carrier, reduces the likelihood of injury to personnel since the terminal is retained on the flexible tail 17. On the contrary, the conventional fuse link loses its lower terminal on high fault levels and may result in injury to unprotected personnel.

The fault rating of the fuse link is varied by varying the number of wires 20 used in the fusible element 10 or by varying their diameters. However, the diameter of the wire must be sufficiently small to ensure vapourization or product of very small particles on fusing. It is considered that the wire diameter may vary between 0.002 and 0.010 inch and still produce reasonable results. The ABS collet may be replaced by another suitable collet, for example a collet of Delrin (Registered Trade Mark) or other acetyl copolymers and plastics.

The embodiment described above is purely exemplary and many modifications within the scope of the appended claims may be made by persons skilled in the art. For example, the wires 20 forming the fusible element may be coated as a bundle, the performance still being substantially identical to the described arrangement. Alternatively, the wires may be coated and dried and then either brought together in a group or twisted together. Furthermore, the coating may be modified as follows:

1. Natural latex, using lighter fuel as a solvent, may replace Bostik, the proportions of the mixture being similar to the embodiment.

2. Mowital B30l-I (Hoescht) to replace Bostik in similar proportions using methylated spirits as the solvent. To ensure that proper distribution of KC10 is maintained, a material such as Neosyl (a silica compound), or another material which prevents separation, is added in proportions of from 5 to 20 percent by weight.

3. Suitable varnishes and enamels may replace Bostik" in fact any material which burns suitably may be used, a binder or adhesive being mixed to ensure adhesion of the coating to the wires.

4. KNO, may replace KC10 or any other suitable oxygen releasing compound may be satisfactory.

5. A compound which possesses at least the burning and oxygen releasing properties, and a binder or adhesive if necessary.

The performance of alternatives 1 and 2 is substantially the same as the above embodiment. The temperatures at which burning and oxygen release occurs is not critical: they may occur at temperatures approaching silver fusion or at temperatures which occur once the arc is created.

The wires 20 may be replaced by other silver element(s) such as thin silver strips or tubes of the required effective cross-sectional areas to enhance their vapourizing properties. In the case of a tube 30, as seen in FIG. 4, the coating 32 may be applied on the inside and/or outside surface thereof.

It may be possible, as seen in FIG. 5, as an alternative to coating, to form tubes 34 or helical strips of material having the required burning and combustion supporting properties and which would surround the silver element(s) 36. Furthermore the constituents performing to two necessary functions may be separated, although experiments have shown that best results are achieved when intimate mixtures are used.

The fiber tube 8 may be formed of other materials and constructions, although vulcanized fiber is preferred for its gas forming properties and spiral winding is preferred for its rupture characteristics and high tensile strength.

We claim:

I. A fuse link for a gas expulsion fuse comprising a top arcing terminal and a bottom arcing terminal, a fusible element electrically connecting said terminals, said fusible element comprising one or more silver elements of small effective cross-sectional dimension which tend to vaporise, or form very small particles which cool rapidly, on fusion resulting from faults at current distribution voltages, the element or elements being coated or having arranged in close proximity thereto a compound or mixture which burns and which produces oxygen at temperatures approaching or at the fusion temperature or at temperatures which occur once an arc is produced by rupture of the element or elements to facilitate the vaporisation of said element or elements, a tube enclosing said fusible element and in the ends of which said terminals are received, means for firmly securing said top arcing terminal in said tube, and means for less firmly securing said bottom arcing terminal in said tube to allow the usual forces applied to said terminals to be supported by the tube rather than by the fusible element but to allow said bottom arcing terminal being expelled from said tube by the gas pressure created in said tube on occurrence of a fault.

2. An element according to claim 1 wherein said silver element or elements are fuse wires or strips which are coated individually or in a group.

3. An element according to claim 2, wherein the fuse wires are from 0.002 to 0.0l0 inches in diameter, preferably 0.003 inches, each wire being coated and either formed into a group before the coating is dry or grouped or twisted together after the coating is dry.

4. An element according to claim 1, wherein said silver element is a thin silver tube coated on a substantial part or whole of its inside surface and/or outside surface.

5. An element according to claim 1, wherein the coating comprises a mixture of rubber based cement or latex or varnish or a varnish-like compound or enamel, and KCl0 or KNO mixed with a suitable solvent to form a slurry, with the KCl0 or KNO dispersed throughout the mixture.

6. An element according to claim 1, wherein the fusible element is substantially surrounded by a member formed from or impregnated with said mixture or compound; if impregnated the carrier being capable of disintegration burning during a fault.

7. A fuse link according to claim 1, wherein said tube is composed of spirally wound vulcanized fibre which gives off a gas on heating of temperatures encountered on faults occurring.

8. A fuse link as claimed in claim 1, wherein the means for securing said top arcing terminal comprises a metal collet surrounding the tube and a tight fit thereon, and the means for securing the bottom arcing terminal in a collet of plastics material which facilitates expulsion of this terminal.

9. A fuse link as claimed in claim 8, wherein the coating comprises a mixture of rubber based cement or latex or varnish or a varnish-like compound or enamel, and KC10 or KNO mixed with a suitable solvent to form a slurry, with the KC 10:; or KNO dispersed throughout the mixture.

10. A fuse link as claimed in claim 1, wherein the bottom arcing terminal is constructed to have a reduced mass to limit its expulsion velocity. 

1. A fuse link for a gas expulsion fuse comprising a top arcing terminal and a bottom arcing terminal, a fusible element electrically connecting said terminals, said fusible element comprising one or more silver elements of small effective crosssectional dimension which tend to vaporise, or form very small particles which cool rapidly, on fusion resulting from faults at current distribution voltages, the element or elements being coated or having arranged in close proximity thereto a compound or mixture which burns and which produces oxygen at temperatures approaching or at the fusion temperature or at temperatures which occur once an arc is produced by rupture of the element or elements to facilitate the vaporisation of said element oR elements, a tube enclosing said fusible element and in the ends of which said terminals are received, means for firmly securing said top arcing terminal in said tube, and means for less firmly securing said bottom arcing terminal in said tube to allow the usual forces applied to said terminals to be supported by the tube rather than by the fusible element but to allow said bottom arcing terminal being expelled from said tube by the gas pressure created in said tube on occurrence of a fault.
 2. An element according to claim 1 wherein said silver element or elements are fuse wires or strips which are coated individually or in a group.
 3. An element according to claim 2, wherein the fuse wires are from 0.002 to 0.010 inches in diameter, preferably 0.003 inches, each wire being coated and either formed into a group before the coating is dry or grouped or twisted together after the coating is dry.
 4. An element according to claim 1, wherein said silver element is a thin silver tube coated on a substantial part or whole of its inside surface and/or outside surface.
 5. An element according to claim 1, wherein the coating comprises a mixture of rubber based cement or latex or varnish or a varnish-like compound or enamel, and KC103 or KNO3 mixed with a suitable solvent to form a slurry, with the KC103 or KNO3 dispersed throughout the mixture.
 6. An element according to claim 1, wherein the fusible element is substantially surrounded by a member formed from or impregnated with said mixture or compound; if impregnated the carrier being capable of disintegration burning during a fault.
 7. A fuse link according to claim 1, wherein said tube is composed of spirally wound vulcanized fibre which gives off a gas on heating of temperatures encountered on faults occurring.
 8. A fuse link as claimed in claim 1, wherein the means for securing said top arcing terminal comprises a metal collet surrounding the tube and a tight fit thereon, and the means for securing the bottom arcing terminal in a collet of plastics material which facilitates expulsion of this terminal.
 9. A fuse link as claimed in claim 8, wherein the coating comprises a mixture of rubber based cement or latex or varnish or a varnish-like compound or enamel, and KC103 or KNO3 mixed with a suitable solvent to form a slurry, with the KC103 or KNO3 dispersed throughout the mixture.
 10. A fuse link as claimed in claim 1, wherein the bottom arcing terminal is constructed to have a reduced mass to limit its expulsion velocity. 